Extended aromatic ring systems

The chiral distinction capability of cinchonan carbamate CSPs for underivatized amino acids has not been fully elucidated yet, in contrast to the large embodiment of A-acylated and A-arylated amino acid derivatives vide infra). However, it seems that chiral amino acids can be successfully resolved into enantiomers if the amino acid side chain R residue) contains a functionality that represents a strongly interactive binding site with the selector such as an extended aromatic ring system like in thyroxin (T4). 

The substituted [2.2]paracyclophanes prepared by the present procedure have proven to be useful starting materials for the synthesis of cyclophanes with extended aromatic ring systems,16 additional ethano bridges,17-18 and chromium tricarbonyl complexes.19... 

Calculations with one isolated molecule in vacuum often result in overestimation of intramolecular contacts, however, because competing interactions are absent. We started our investigation with the semi-rigid guanidinothiazole ICI27032, a potent competitive H2-antagonist which adopts solely extended conformations, because of its aromatic ring system (Fig. 9.8). 

Resonance-stabilized systems include car-boxylate groups, as in formate aliphatic hydrocarbons with conjugated double bonds, such as 1,3-butadiene and the systems known as aromatic ring systems. The best-known aromatic compound is benzene, which has six delocalized k electrons in its ring. Extended resonance systems with 10 or more 71 electrons absorb light within the visible spectrum and are therefore colored. This group includes the aliphatic carotenoids (see p.l32), for example, as well as the heme group, in which 18 k electrons occupy an extended molecular orbital (see p. 106). 

On the basis of the structural requirements for an efficient modifier, and using the above model as a working hypothesis, several new modifiers have been developed (Fig. 3) [6,29,30], These studies revealed that (i) only extended flat aromatic ring systems (the naphthyl, quinolyl, anthracenyl groups in 1-3) are efficient anchoring moieties, whereas phenyl or pyridyl rings in 4 or the non-flat triptycenyl moiety in 5 are not suitable for adsorption parallel to the metal surface, and (ii) the N-heteroatom in the aromatic ring system (2) is not a necessary requirement for adsorption of the modifier and enantio-differentiation. 

Rabinovitz and co-workers also used the Haworth phenanthrene synthesis to form complex fused aromatic ring systems." The bowl-like shape of corannulene 57 was extended through the acylation reaction with 58 to produce 59. Treatment of 59 with red phosphorus under strongly acidic conditions resulted in 60 in good yield. 

---